Direct chip attachment (DCA) refers to a semiconductor assembly technology wherein an integrated circuit chip is directly mounted on and electrically connected to its final circuit substrate instead of undergoing traditional assembly and packaging. Advantageously, the elimination of conventional device packaging in DCA both simplifies the manufacturing process and reduces the space that the integrated circuit chip occupies on the final circuit substrate. It also improves performance as a result of the shorter interconnection paths between the integrated circuit chip and the circuit substrate.
Flip chip attachment to a flexible circuit substrate (flip-chip-on-flex (FCoF)) is one variant of DCA that is evolving into a mainstream process for the construction of some classes of electronic devices. A flip chip comprises an integrated circuit chip with a multiplicity of conductive solder bumps attached to the chip's bonding pads. In FCoF, these solder bumps are directly attached to a flexible circuit substrate. The flexible circuit substrate, in turn, comprises copper electrical traces that act to connect the flip chip with other electronic components in a manor similar to a conventional rigid printed circuit board. Flexible circuit substrates are used extensively in applications where their unique ability to bend and curve into flexible shapes is required. Flexible circuit substrates are used, for example, in notebook computers, hard disk drives, PCMCIA (Personal Computer Memory Card International Association) connectors, docking stations, pointing devices, compact disk players and mobile phones.
Mounting a flip chip on a flexible circuit substrate involves attaching the solder bumps on the flip chip to the copper electrical traces incorporated into the flexible circuit substrate. In a flexible circuit substrate, these copper electrical traces are typically covered with some kind of polymer coverlayer, frequently called a “solder mask.” Accordingly, openings are created in the polymer coverlayer to create attachment areas that allow the solder bumps on the flip chip to access the underlying copper electrical traces. A high temperature reflow process is then used to permanently attach the solder bumps to the copper electrical traces. Subsequently, a non-conductive underfill material is dispensed into the region between the flip chip and the flexible circuit substrate. The underfill material protects the solder bumps from moisture and other environmental hazards, provides additional mechanical strength to the assembly, and compensates for any thermal expansion difference between the flip chip and the flexible circuit substrate.
Because of its many functions, the underfill material will preferably completely and uniformly fill the region between the flip chip and the flexible circuit substrate. This requires that the distance between the flip chip and flexible circuit substrate (“standoff distance”) be precisely controlled. Unfortunately, in conventional FCoF applications, this standoff distance is influenced by many factors including the size of the attachment area on the flexible circuit substrate, the thickness of the polymer coverlayer (i.e., solder mask), the ductility of the solder bump alloy and the reflow profile of the solder bumps. For example, if a large attachment area is exposed on the flexible circuit substrate, a solder bump landing in that area will have a tendency to wick along the exposed copper electrical traces during the high temperature reflow process. This causes the standoff distance to decrease, compromising the application of a complete and uniform layer of underfill material between the flip chip and the flexible circuit substrate.
As a result, there is a need for an improved flexible circuit substrate for use in FCoF applications that promotes a larger and more uniform standoff distance between the flip chip and the flexible circuit substrate when compared to conventional flexible circuit substrates.